Interrelation of Polynuclear Aromatic Hydrocarbons and

A sketch of Debye-Scherrer X-ray diffraction photograph of C8K made from .... The C~ 1 6 K + ( H 2 0 ) 2 5 proposal of Ebert and co-workers (41-. 43) ...
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21 Interrelation of Polynuclear Aromatic Hydrocarbons and Carbonaceous Materials Lawrence B. Ebert and Joseph C. Scanlon Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801

Themes linking polynuclear aromatic hydrocarbons to solid carbonaceous materials, including fossil fuels and graphite, are explored. X-ray diffraction proves that one may intercalate metaanthracite coal and calcined petroleum coke in the same sense that graphite may be intercalated. The chemistry of reductive protonation of large aromatic crystallites is investigated.

T H E TERM "AROMATIC" HYDROCARBON originates from the existence of many pleasant-smelling, naturally occurring oils, such as those from cinnamon, vanilla, and cloves, which contain molecules related to benzene. Polynuclear aromatic hydrocarbons are also found in nature, most frequently in coal and petroleum. A book on the chemistry of polynuclear aromatic hydrocarbons thus can generate themes relevant to the chemistry of fossil fuels, and this idea is developed in several chapters in this book (1-4). In this final chapter, we shall discuss issues linking the chemistry of polynuclear aromatic hydrocarbons, fossil fuels, and the ultimate aromatic compound, graphite. In studying this interface, one sees the relevance of the organic chemistry of pure compounds and the possible utility of the materials science of carbonaceous solids in learning more of the chemistry of fossil fuels. The relation between polynuclear aromatic hydrocarbons and fossil fuels is discussed in introductory organic chemistry textbooks, which mention that coal and petroleum are the commercial sources of many aromatic hydrocarbons (5, 6). The chemistry discussed in these basic texts is generally limited

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In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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to substitution reactions (electrophilic and nucleophilic, with no change i n hybridization of carbon atoms) and addition reactions (such as the oxidative addition of bromine or the reductive addition of hydrogen to yield some carbon atoms that are sp hybridized). These texts point out that polynuclear aromatic hydrocarbons are more reactive than benzene (5, 6). The concept of reactivity of the ττ electrons can be quantified by measurement of ionization potential (IP) and electron affinity (EA), which also allows comparison of molecular species with graphite. Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 31, 2014 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch021

3

Graphite as a Polynuclear Aromatic

"Hydrocarbon'

As the size of polynuclear aromatic hydrocarbons increases, a gradual decline in IP and an increase in E A occur; these changes culminate i n graphite, for which IP = E A = 4.39 electronvolts (eV) (7). This trend suggests that larger fused-ring aromatic hydrocarbons will be more reactive than smaller ones in reactions involving electron transfer, whether the reactions involve elec­ tron addition (reduction) to form aromatic anions or electron removal (oxi­ dation) to form aromatic cations. This concept of a hierarchy in electrontransfer reactivity has been appreciated in work on both model compounds (8) and on coals (9). Additionally, one sees that graphite, far from being inert, is actually more reactive to both reduction and oxidation than most aromatic hydrocarbons; in fact, ions of polynuclear aromatic hydrocarbons will react with graphite via electron transfer to form intercalation compounds (10). The increasing importance of electron-transfer reactions with increasing aromatic hydrocarbon size is illustrated i n the reaction of bromine with various aromatic compounds. W i t h benzene (with a Lewis acid) and with naphthalene, electrophilic substitution occurs, and with anthracene, oxi­ dative addition occurs (6); however, with graphite, only oxidation to the exclusion of carbon-bromine bond formation occurs, even at a stoichiometry of C B r (II, 12). Graphite translational symmetry elements can be related to reactivity classes of aromatic hydrocarbon molecules. As illustrated i n Figure 1, the acene class of aromatic hydrocarbons (e.g., anthracene, naphthacene, and pentacene) can be defined in terms of growth along the graphite [110] d i ­ rection of translational periodicity. The phenacene class of aromatic hydro­ carbons (e.g., phenanthrene, chrysene, and picene) can be defined in terms of growth along the graphite [100] direction of translational periodicity, although detailed X-ray diffraction studies show that phenacenes, in contrast with graphite, are slightly nonplanar (13). The reactivity of acenes to either reduction or oxidation increases rapidly with molecular size, whereas the reactivity of phenacenes changes rather slowly (7). For a given molecular size, an acene is more reactive than a phenacene, and, in this book, Nishioka and Lee (4) discuss differences i n reactivity between phenanthrene and anthracene i n the hydrogénation of a coal liquid. 8

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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duo

- 123

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pm

Figure 1. Illustration of the acene and phenacene molecular classes defined in terms of graphite translational periodicity axes. In terms of the phenacenes, by starting at the lower left, one may generate phenanthrene (three rings), chrysene (four rings), and picene (five rings).

The interrelation between molecular reactivity and translational peri­ odicity can have experimental implications. In benzenoid solids, one ob­ serves X-ray diffraction peaks arising from periodicity along the (110) direction at distance d = 1.23 Â (123 pm) and from periodicity along the (100) direction at d = 2.13 A (213 pm). For a material with no preferred growth direction, such as graphite, the crystallite size predicted by using the Scherrer equation evaluation of diffraction line width (14), will be com­ parable for both peaks. However, if a direction of growth is preferred, caused perhaps by preferred sites of molecular reactivity, unequal crystallite sizes from the (110) and (100) line widths will be seen (7, 15). Such an analysis should be relevant to various ladder polymers (16-19) and to the proposal of chiral graphite (20). Ruland and Plaetschke (21) have outlined a diffraction method to determine preferred (100) and (110) orientation in carbon fibers arising from the pyrolysis of different benzenoid materials (21). Significantly, fibers derived from polyacrylonitrile ladder polymer, which is an acene, had the opposite preferred orientation to fibers derived from petroleum mesophase pitch, which presumably is composed primarily of phenacenes (2, 4). The interrelation of initial molecular symmetry and reactivity with final structure is an important issue in fossil fuels, as in the pyrolysis of coal or petroleum to make coke. In the general area of pyrolysis of aromatic hy­ drocarbons, several reviews have been published (22-24), and in this book, Lewis and Singer discuss recent advances in this field (25).

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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Coal and petroleum are more complex than simple pure polynuclear aromatic hydrocarbons or graphite because the fossil fuels contain not only aliphatic groups, but also elements other than carbon and hydrogen. Unlike many of the polynuclear aromatic compounds discussed in this book, coal and petroleum are not fully soluble in common organic solvents partly be­ cause of cross-linking through aliphatic groups and partly because of the effect of the other elements, such as sulfur, nitrogen, and oxygen. Thus, a chemical reaction that is predicated on the solvation of the converted aro­ matic hydrocarbon, such as many of the reductive chemistries discussed in this book, may not be completely effective in reacting with the fossil fuels. A n alternative starting point in chemically reacting fossil fuels is to treat them as if they were graphite. As noted earlier, graphite and larger poly­ nuclear aromatic hydrocarbons are far from inert with respect to electrontransfer reactions, and thus the use of chemistry known to work for graphite may be of possible use in the investigation of coal, petroleum, and their derivatives. In the next two sections, we will discuss aspects of reduction and oxidation of carbonaceous solids and thereby parallel the chapters in this book on the reduction and oxidation of polynuclear aromatic hydrocarbon molecules.

Reduction of Carbonaceous Solids F o r m a t i o n of the G r a p h i t e I n t e r c a l a t i o n C o m p o u n d C K. Graphite will react directly with potassium metal at 110 °C under inert gas to form a bronze-colored intercalation compound of stoichiometry C K (26). This compound is first-stage, meaning that every interlayer void in the graphite is filled with potassium. (Stage is defined as the ratio of graphite layers to guest layers.) This condition is demonstrated by X-ray diffraction. A sketch of a Debye-Scherrer photograph of a polycrystalline graphite C K compound is shown in Figure 2; the lines readily index to an orthohexagonal Bravais lattice of a = 4.961 Â, b = 8.592 Â = V3Ô^, and c = 21.76 Â, or to the equivalent hexagonal lattice of a = 4.961 Â and c = 21.76 Â (10, 11). Figure 3 presents variable-temperature Guinier-Simon X-ray diffraction data that show that the C K intercalation compound is destroyed by heating under a nitrogen flow at 500 °C. 8

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Formation of C K from Metaanthracite Coal. This relatively simple chemistry can also cause intercalation of nongraphitic, but benzenoid, carbonaceous solids. Figure 4 presents a Debye-Scherrer X-ray diffraction photograph of a polycrystalline C K compound formed by the insertion of potassium metal at 110 °C into a metaanthracite from Newport County, RI (27). A sketch of this photograph is also shown in Figure 4. The initial crystallite size along the c axis, the direction of stacking, was 124 A , as measured from the line width of the (002) line at d = 3.36 A . The most 8

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{Ni filter}-

Miller Indices:

ο ο

CM

t

tt

5 ο ο

~ ~ ο ο τ- ο

C Ks a 8

c

0

0

t

t

5.35À 2.70À of C K lattice 8

• 4.9À - 21.40À

( - 4 χ 5.35À)

Figure 2. A sketch of Debye-Scherrer X-ray diffraction photograph of C K made from polycrystalline graphite and potassium metal. The indexing is done with respect to the hexagonal Bravais lattice of C K, which is degenerate with the orthohexagonal lattice. In the figure, the peak labelled (101) is at 4.2 Â; this index is equivalent to the (111) peak of the orthohexagonal system. It is likely that the broad 4.2-Â peak also represents the (100) of the hexagonal lattice (equal to the (110) of the orthohexagonal system). The peak labelled (200) in the figure is at 2.1 Â this index is equivalent to both the (200) and (040) peaks of the orthohexagonal system. 8

8

Ο Ο Ο

ο

ω ο ο

τO

τ- τ­

ο

CM

C K Lines (Hex. Lat.) 8

2Θ 25°C (Start) 100°C < 500°C (Hold)

3 CO

k . Ο

α Ε α> Η

100°C (Finish)

Graphite Lines

CM Ο Ο

Ο Ο

τΟ

Ο

ο

Figure 3. Variable-temperature X-ray diffraction of C K inflowing nitrogen. The temperature increases from 100 to 500 °C and then decreases back to 100 °C. 8

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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Figure 4. Debye-Scherrer X-ray diffraction photograph of C K made from metaanthracite coal and potassium metal (top), and sketch of this photograph (bottom). 8

intense lines of the product of the reaction with potassium are at 5.3, 4.2, 2.7, and 2.2 A , which index as the (004), (111) [and possibly (110)], (008), and (220) lines of the orthohexagonal C K Bravais lattice. Diffraction peaks still occur at d = 3.36 À and d = 1.68 A , which could be the (002) and (004) lines of the initial coal; however, the intensity ratio of the peak at d = 5.3 Â [the (004) line of the C K ] to the peak at 1.68 Â [maybe the (004) line of the initial coal] is 10.8; this ratio suggests that the intercalated product is the principal product. For an equal weight mixture of graphite C K and graphite, this intensity ratio is 1.6. A n additional argument in favor of the presence of intercalation is the temperature dependence of the line width of the electron spin resonance signal. Although the starting metaanthracite shows a derivative extremum line width of 12.5 ± 1.0 gauss (G) that is independent of temperature, the C K product shows a line width that depends upon temperature: 8

8

8

8

ΔΗ (gauss) = 0.0354Γ + 14.28 R = 0.9500 2

(1)

where Τ is the temperature in kelvin. This linear dependence is similar to that reported for the graphite C K compound (28) and is the behavior ex­ pected for metallic conductors. 8

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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The ability of metaanthracite coal to intercalate potassium metal is pre­ sumably related, in part, to its large crystallite size and to its low H / C ratio (H/C = 0.036; 93.29% C , 0.28% H ; dry basis). F o r an anthracite coal (Wilkes-Barre, PA) with a crystallite size of 15 A and an H / C ratio of 0.30 (83.31% C , 2.11% H), as well as for a low-volatile bituminous coal (Ireland mine, WV) with no measurable stacking and an H / C ratio of 0.83 (69.10% C , 4.83% H), the reaction with potassium at 110 °C gave no evidence for the formation of an intercalation compound. Formation of C K from Calcined Petroleum Coke. Materials that are less crystalline than metaanthracite do form intercalation com­ pounds. Calcined petroleum cokes are formed by thermolysis at approxi­ mately 1400 °C of "green" petroleum cokes, which in turn are made by thermolysis of aromatic petroleum streams i n delayed cokers operating in the range of 400-500 °C (29-32). (Immature cokes of higher H / C ratio are termed "green"; they are not green in color.) The crystallite size, measured both in the direction of aromatic stacking (c axis) and in the in-plane direction (a axis) is of the order 50-60 A , which is less than half the crystallite size of the metaanthracite. However, literature reports indicate that such ma­ terials do form intercalation compounds with potassium metal (33, 34). By reacting a commercially available calcined coke with potassium metal at 110 °C, we confirmed the ability of such cokes to intercalate potassium (35). Figure 5 presents a Debye-Scherrer X-ray diffraction photograph of a C K product formed from a calcined coke from a different manufacturer. A sketch of this photograph is also shown in Figure 5. This second coke had 8

8

Figure 5. Debye-Scherrer X-ray diffraction photograph of C K made from calcined petroleum coke and potassium metal (top), and sketch of this photograph (bottom). 8

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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crystallite sizes similar to the first coke and a comparable low H / C ratio of 0.013 (98.95% C , 0.11% H); the sulfur level was 0.45%.

Protonation and Rehybridization of Aromatic Anions. These intercalation compounds of benzenoid, but nongraphitic, materials can be of relevance to themes in fossil-fuel chemistry. One challenging proposal is that chemistries of reduction, followed by protonation, can cause hydrogen uptake and rehybridization of sp carbon both in coals and in graphite. Specifically, X-ray scattering studies of seven vitrains and a graphite sug­ gested "that conversion of planar aromatic layers to buckled hydrogenated rings had taken place", following treatment with lithium in ethylenediamine (36). The studies also reported that, "even with graphite of different origins, hydrogen increases of up to about 30 hydrogens per 100 carbon atoms were obtained", following reduction with alkali metal in glycol ethers and a quench with protonating reagents (36). Such reports are related to work in graphite science in which the protonation of the graphite intercalation compound C K with water was stated to lead to the new intercalation compound C H (37, 38). We shall discuss the plausibility of the two reductions of graphite by solution-state approaches and then give new data pertinent to the C K - H 0 reaction. 2

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The claims for the rehybridization of graphite in the solution-state ap­ proaches were based on the following: (1) major changes in the X-ray dif­ fraction at low scattering angles, and (2) the increase in hydrogen content of the reduced product. The reaction of graphite with alkali metal in amine solvents to yield intercalation compounds is known in the literature (10). In such reactions, which are unaccompanied by carbon rehybridization, intercalation causes major changes in X-ray diffraction at low scattering angles, and for concen­ trated compounds, the most intense peak will be at angles lower than the initial graphite (002) line (10). Solvents typically cointercalate with the alkali metal, and in the specific case of graphite reacting with lithium and eth­ ylenediamine, 1 mol of ethylenediamine intercalates per mole of intercalated lithium (10). The behavior of graphite with alkali metals and various ethers is similar (39). Thus, one sees that the changes observed in graphite upon reduction in the liquid phase can be accounted for on the basis of intercalation without invoking any rehybridization of graphitic sp carbon atoms to sp carbons. Furthermore, if a two-dimensional polymeric ( C - H ) network of naphthenic carbon were generated, one could predict that the principal basal-plane diffraction peak would occur at 5.2 Â on the basis of the known structure of the two-dimensional fluoronaphthene, namely poly(carbon monofluoride) (15, 40). 2

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Reaction of C K Made from Graphite with Water. For the reaction of graphite C K with water, three different models are proposed for the product: 8

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1. The C H proposal of Bergbreiter and Killough (37, 38) in which C K is assumed to react much like an anion of a poly­ nuclear aromatic hydrocarbon. 8

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2. The C ~ K ( H 0 ) proposal of Ebert and co-workers (4143) in which a fraction of the intercalated potassium in C K behaves as potassium metal and in which the graphitic macrocarbanion is resistant to protonation. The product is con­ sidered to be a poorly crystalline intercalation compound containing Κ and water. This model serves as a counterpoint to a model assuming rehybridization of carbon, as discussed in the preceding section. +

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1 6

2

2 5

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+

3. The graphite and K O H model of Schlogl and Boehm (44) in which poorly crystalline graphite encapsulates potassium hy­ droxide. The graphite is uncharged, and the K O H (or perhaps K O H monohydrate) exists in crystallites observable (at least in part) by X-ray diffraction. Although Schlogl and Boehm (44) refer to their product as a residue compound, it is not a residue compound in the commonly accepted sense because it contains the elements H and O, which do not occur in the initial parent intercalation compound. Although proposal 1 is most closely related to the liquid-state reductions discussed earlier, the current debate in the graphite community is between proposals 2 and 3; proposal 1 is currently discounted. However, although proposal 1 may be of marginal significance for the large crystallites charac­ teristic of graphite, the situation with intercalated carbonaceous material of smaller crystallite size is unclear. For this reason, we discuss the reaction of water with C K made from petroleum cokes at 1400 °C. 8

Reaction of C K Made from Petroleum Coke with Water. The coke discussed in reference 35 contained 99.40% C , 0.17% H , and 0.31% S. In the presence of a stoichiometric amount of potassium at 110 °C, 5.693 g of the coke (471 mmol of C) reacted with 2.342 g (59.9 mmol) of potassium to form bronze-colored C K . Under helium, 3.048 g of this product was reacted with excess H 0 , thoroughly rinsed, and dried to constant mass at 110 °C under a 5 0 ^ m H g vacuum. Hydrogen gas was evolved on contacting this C K with water. The yield of solid was 2.392 g solid of analysis 89.66% C, 5.94% K, 0.96% H , 0.22% S, and 3.26% O. The empirical formula of the product is thus Ο^ΚΗ^Οχ. S . 8

8

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0>045

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

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To get a true picture of the final H / O atomic ratio, we must correct for the hydrogen initially in the coke (the initial amount of oxygen is more than an order of magnitude smaller than the final amount, so we will ignore it). The 3.048 g of the C K contained 2.161 g of coke, which contained 2.148 g of C , 3.67 mg of H , and 6.70 mg of S. The 2.392 g of recovered product contained 2.145 g of C (99.3% recovery of carbon!), 0.142 g of K, 0.02296 g of H , 0.00526 g of S, and 0.07798 g of Ο. The H / O ratio of added material is [(0.02296 - 0.00367)/1.008]/(0.07798/15.9994) = 3.93. If the only reaction of this coke were the intercalation of water, the H/O ratio should be 2, as it is in the case of the product of the reaction of graphite C K with water. If we were dealing with the encapsulation of K O H monohydrate, the H / O ratio should be 2.

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Frontier between Polynuclear Aromatic Compounds and Graphite. The calcined petroleum coke shows chemistry that contains components of both graphite and polynuclear aromatic hydrocarbons. Like graphite, the calcined petroleum coke forms first-stage intercalation com­ pounds with both potassium and cesium metals. These compounds are iden-

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tified by the presence of the correct Bravais lattice, as shown by X-ray diffraction, and by the absence of the initial Bravais lattice. Also, upon reaction with water followed by rinsing, not all of the initial alkali metal can be removed from the calcined petroleum coke. Furthermore, there is evi­ dence for the incorporation of water in the final solid-phase product. O n the other hand, like polynuclear aromatic hydrocarbons, the anions from the coke react with water to (possibly) generate C - H bonds. Where does the frontier between large polynuclear aromatic hydrocar­ bons and graphite lie? The answer is difficult to determine. Coronene, C H , reacts with potassium at 110 °C to form a solid black product with an electron spin resonance signal whose line width depends linearly on temperature but whose area follows the Curie law (7). Neither X-ray dif­ fraction nor intuition suggests that this product be called an intercalation compound, yet the electron spin resonance indicates that potassium metal can react with larger polynuclear aromatic compounds in the absence of a solvent (7, 45, 46). Thus, a gradual transition in chemistry may occur from solvated aromatic anions (such as the naphthalene radical anion in tetrahydrofuran) to nonsolvated localized aromatic anions [such as coronene with K(0)] to nonsolvated delocalized anions [such as graphite with K(0)]. In general, the intercalation of benzenoid, but nongraphitic, solids with chemical reductants has been studied more than the intercalation of these solids with chemical oxidants. Herold (47) suggested that soft carbons, such as those obtained from petroleum, have acceptor defects, which favor the addition of electrons (reduction) to the benzenoid structure. Hooley (48) discussed the divergences in terms of threshold pressure effects. Neverthe­ less, the oxidation of carbonaceous solids has been studied, and these studies are discussed i n the next section. 2 4

1 2

Oxidation of Carbonaceous Solids Early approaches to the oxidation of carbonaceous solids were destructive in nature and led to C 0 and possibly to organic acids (49-52). More recently, the oxidative intercalation of nongraphitic materials was reported (53-55). Lazarov et al. (54) reacted a bituminous coal from Bulgaria (87.2% C , dry ash free) with several metal chlorides in carbon tetrachloride. The con­ ditions used were those known to make intercalation compounds of graphite, and the authors reported that rather concentrated adducts formed with S b C l , A1C1 , F e C l , and M o C l . No (002) band of the initial coal was ob­ served in the adducts; this finding is consistent with the presence of inter­ calation. Coals of bituminous ranks show only a short-range order of aromatic stacking; the typical arrangement is only two to three layers (7-10.5 Â), and a liquidlike structure is considered to be optimum at 89% C (dry ash free) (56). Thus, in contrast with the petroleum cokes, the diffraction pattern of the final intercalation compound could be difficult to see. Furthermore, other 2

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complexities are possible in the diffraction, including the presence of other peaks (57) and distributions of dihedral angles between aromatic planes (58). In general, aromatic hydrocarbons and presumably fossil fuels contain­ ing aromatic molecules do not form extended "stacks" of molecules as em­ bodied by a column of poker chips. H - H and H - C interactions are significant structural determinants for hydrocarbon molecules (59-60) and can lead to arrangements other than the graphitelike stack. To see a diffraction peak in the range of 3.3-3.8 Â and infer a graphite structure is potentially naive. For petroleum cokes formed in the temperature range of 1600-2800 °C, X-ray diffraction was useful in monitoring the oxidative intercalation by S b C l , B r , and C r 0 - a c e t i c acid (55). With crystallite sizes in the direction of stacking between 300 and 600 A , these cokes behave much as graphite with respect to oxidative intercalation; they even show such subtle behavior as stage disorder (analogous to graphite) in the case of the S b C l product. Furthermore, electron microprobe data on the C r 0 adduct showed an irregular C r concentration profile, which suggested that intercalation oc­ curred only at the edges of crystallites and did not proceed to the interior. This new result on an intercalation compound of coke is quite relevant to understanding the analogous graphite compound. F o r the graphite com­ pound, diffraction results suggested the presence of both intercalated and nonintercalated phases, and magnetic resonance suggested that the inter­ calated chromium species, in contrast with most intercalated species, was immobile (61). Combining the information from both coke and graphite, we see that this intercalation system is rather special; the inability of the inserted species to be translationally free precludes the complete intercalation of the interlamellar voids. 5

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In several studies, researchers attempted both the reduction and oxi­ dation of carbonaceous solids (48, 54, 62, 63). One group claimed that graph­ ite, oxidatively intercalated by C d C l , can be later reductively intercalated by sodium (64). In any event, various chemistries will attack both graphite and carbonaceous solids, and various physical techniques can prove this attack. 2

The Future In studying both polynuclear aromatic hydrocarbons and carbonaceous sol­ ids, X-ray diffraction has proved to be a valuable tool; this book contains elegant structural studies on both aromatic anions (65) and aromatic cations (66). N M R , including both C N M R in the solid state (67-72) and various two-dimensional spectroscopies (73-75), will have a dramatic impact in the future on both polynuclear aromatic hydrocarbons and carbonaceous solids. The materials to be studied promise to be ever more challenging, rang­ ing from soots and combustion exhausts (76-78), to mesophase materials from petroleum (79), through the graphite-diamond interface (80-81), antiaromatic compounds (82), carbon clusters (83, 84), and interstellar materials 1 3

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(85-89). Furthermore, work on aromatic hydrocarbons should prove of rel­ evance i n other areas such as electrochemistry (90), gasification (91), and carbon reactivity in general (92-95). Finally, aromatic hydrocarbons them­ selves continue to provide exciting chemistry (96-98).

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Note Added in Proof We were unable to cover some areas related to polynuclear aromatic mol­ ecules that are currently attracting attention. Discotic liquids crystals (also termed columnar) are frequently composed of a central core composed of a polynuclear atomatic molecule substituted with pendant paraffin chains. Although liquids, they have crystalline order in certain directions, as shown by narrow X-ray diffraction peaks. Brown and Crooker wrote a basic discussion (99) and Hinov compiled a literature review for the period 1977-1984 (100). Other papers show the relation to polynuclear aromatics (101-110). One usually does not relate polynuclear aromatic compounds to dia­ monds, yet recent work on dense, "diamondlike" films shows the presence of sp -hybridized carbons (111-113). Thus, the chemistry of polynuclear aromatic compounds may be of relevance to this exciting new materials area. Finally, much more needs to be done in interrelating the chemistry of aromatic molecules to the chemistry of carbonaceous solids. A comparison of the chemistry of dibenzothiophene to the chemistry of sulfur functionality in petroleum residues recently was published (114) as well as a comparison of soot to polynuclear aromatics (115). 2

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RECEIVED

for review January 19, 1987.ACCEPTEDMarch 11, 1987.

In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.